A Nato helicopter attacked a Pakistani army post near the Afghan border, injuring two Pakistani soldiers in an incident that could further increase tensions following the US raid that killed Osama bin Laden, Pakistani intelligence officials said.

Nato confirmed there were helicopters flying near the Pakistani border and was investigating the reported attack.

"There were helicopters operating in the border region, and we are aware there has been an incident," said Nato coalition spokesman Lt. Col. John Dorrian. "But we are going to have to assess the situation."

He declined to give further details or say which Nato country was involved.

Most of the helicopters that fly in that part of Afghanistan are American.

Nato helicopters that fly in the area are outfitted with equipment that monitors whether they cross over the Pakistani border, said a Nato official, speaking on condition of anonymity because of the sensitivity of the issue. Authorities are now reviewing that equipment, said the official.

The attack took place Tuesday morning in the Datta Khel area of the North Waziristan tribal region, according to the Pakistani intelligence officials.

Pakistani troops responded with machine gunfire and deployed two helicopter gunships over the post, but the Nato helicopter had already left, they said.

It is unclear how Pakistan will respond if the Nato helicopter attack is confirmed.

Last September, a US helicopter attack killed two Pakistani soldiers at an outpost near the Afghan border, prompting Pakistan to close a key border crossing for 11 days that Nato uses to ship goods into landlocked Afghanistan. The US apologised, saying the pilots mistook the soldiers for insurgents being pursued across the border from Afghanistan.

Relations are even more tense now following the Navy SEAL raid on May 2 that killed bin Laden in Abbottabad, an army town only about 35 miles outside the Pakistani capital, Islamabad.

The Pakistani government is outraged that the U.S. carried out the operation without telling them first, and many U.S. officials have expressed disbelief that bin Laden could have lived in Abbottabad for at least five years without Pakistan knowing. However, the U.S. has also said it has not found any evidence yet that Pakistani leaders knew of bin Laden's whereabouts.

The helicopter attack comes a day after U.S. Sen. John Kerry wrapped up a 24-hour visit to Islamabad in which he worked to salvage the relationship with Pakistan, but also warned the government that "actions not words" were needed to get ties back on track. Kerry was the most high-profile American to visit Pakistan since the raid that killed bin Laden.

Kerry said Pakistan had agreed to immediately take several "specific steps" to improve ties, but did not say what they were. The only tangible signs of progress were a remark by Kerry that Pakistan had agreed to give America the tail of a classified stealth helicopter destroyed by US commandos when it malfunctioned during the raid and an announcement that Secretary of State Hillary Rodham Clinton would soon announce a trip to the country.

But there have also been signs of Pakistan's anger.

The Pakistani government sent the U.S. a written request following the bin Laden raid asking Washington to reduce the number of American military personnel in the country, said a U.S. military official, speaking on condition of anonymity because of the sensitivity of the issue.

There are currently more than 200 U.S. military personnel in Pakistan, some of whom are tasked with training Pakistani troops, said the official. Pakistan has asked the U.S. to reduce the number of trainers in the country, but the official would not specify the numbers involved.

NATO confirmed there were helicopters flying near the Pakistani border and was investigating the reported attack.

A day after the U.S. and Pakistan "resetâ€™â€™ their bilateral relationship, a Yemeni-origin al-Qaeda operative was arrested in Karachi, there was an incursion by two North Atlantic Treaty Organisation (NATO) helicopters into Pakistan â€“ resulting in an exchange of fire â€“ in North Waziristan which also witnessed two drone attacks, and an abortive suicide bomb attack in Quetta involving suspected Chechen terrorists who were gunned down.

If the exchange of fire at the Admi Kot Post in North Waziristan between Pakistani troops and two NATO helicopters threatened to strain the fragile relationship further, the announcement of the arrest of the al-Qaeda operative, Muhammad Ali Qasim Yaqub alias Abu Sohaib Al Makki, should provide Islamabad some breathing space at a time when its commitment to the war on terror is being questioned globally.

The arrest of Makki was announced late in the evening by the Inter Services Public Relations (ISPR) but no details were provided about how he was nabbed. According to ISPR, Makki was working directly under al-Qaeda leaders along the Pakistan-Afghanistan border. However, he is not among Americaâ€™s "most wantedâ€™â€™ as his name does not feature in the U.S. State Departmentâ€™s Rewards for Justice list.

Earlier in the day, the Pakistan Army lodged a strong protest with NATO and demanded a flag meeting over the violation of airspace. Two Pakistani soldiers were injured in the exchange of fire. This coupled with Monday nightâ€™s drone attacks in North Waziristan came just hours after U.S. Senate Foreign Relations Committee chairman John Kerry sought to address Pakistanâ€™s concerns about unilateral action within the country by the Americans as part of their policy of "hot pursuitâ€™â€™. Pakistan, in turn, assured the U.S. that it would act against terrorist sanctuaries in the tribal areas.

Towards late afternoon a major tragedy was averted in Quetta when a check post was attacked by five terrorists â€“ including three women â€“ who were strapped with bombs. According to the police, they were first stopped for checking near the airport but they sped away from the spot. An alert was sounded and they were accosted by the Frontier Constabulary at their check post.

The terrorists then threw hand grenades and fired at them. One of the women blew herself up and all the other terrorists were killed in the firing that ensued. Based on their appearance and documents found on their bodies, security personnel said they were possibly Chechens.